Round-trip Frequency Research Articles

Mode-locked free-electron laser oscillator

Abstract In this experiment we observe emission of a single RF macropulse from a mode-locked FEL oscillator. The FEL employs a continuous electron beam and operates in the microwave regime. The amplitude mode-locking is achieved by modulating the attenuation of the FEL ring cavity at its round-trip frequency ( ∼ 27 MHz ). The observed RF macropulse consists of short RF micropulses. Their period ( ∼ 37 ns ) is determined by the modulating frequency, and their width is comparable to the slippage time ( ∼ 3 ns ). Examples of experimental results of amplitude mode-locked FEL signals are presented for coaxial-line and waveguide cavity sections.

Picosecond pulses (2 ps) from hybrid mode-locked AlGaAs QW lasers with integrated active waveguide cavities

Single ultrashort optical pulses (2 ps with 17.6 GHz repetition rate) have been generated with hybrid mode-locked monolithic GaAs/AlGaAs lasers at lambda =856 nm. Instead of an external cavity, long active waveguides were used as resonators with a roundtrip frequency of 5.9 GHz. Increasing the pump current in the waveguide far above transparency led to 10 ps pulses with a high peak power of 110 mW. >

Integrated Nd:MgO:LiNbO 3 FM mode-locked waveguide laser

Efficient operation of an Nd:MgO:LiNbO3 singlemode waveguide laser with an integrated intracavity electro-optic phase modulator is reported. Pumping the monolithic device at 0-814 μm and driving the modulator at the cavity round-trip frequency resulted in the emission of a continuous train of mode-locked pulses at 1.085 μm with 8ps pulse width, 6.2 GHz repetition rate, and 250 mW peak power for 45 mW of absorbed pump power and 7 V RMS applied.

Instability mechanisms in storage-ring FEL oscillators

The stability of operation for storage-ring FEL oscillators fed by a continuous electron beam is examined. The bunched-beam distribution, at steady state with a large-amplitude carrier signal, is found self-consistently for an arbitrary injected distribution. The dielectric response to small perturbations (sidebands) includes the nonuniformity from aperiodic beam bunching, the anisotropy from the unidirectional interaction with the radiation, and the cross coupling among symmetric upper and lower sidebands caused by the unharmonic (nonlinear) nature of the trapped-particle trajectories. The dominant contribution comes from resonances of the detuning δ = | ωs − ω0| between the sideband and the carrier frequencies, with combinations of the Doppler-upshifted synchrotron (bounce) and round-trip frequencies ωb and Ω respectively, δ; ⋍ 2γ2:(nωb + /Ω). For sideband frequencies corresponding to n ≠ 0, thermal effects are important. The growth rate scales as ωp2 and is proportional to the slope of the distribution across the trapped-particle island. However, the growth of sidebands in direct resonance with the round-trip frequency, δ; ⋍ 2γ2:/Ω, is independent of thermal effects. The growth rate then scales as ωp, and is proportional to the distribution gradients around the trapped-particle island. In effect, the resonance involves a macroparticle rotating with a frequency ωb in harmonic relation with Ω. It. is also shown that in all cases upper and lower sidebands decouple when the sideband growth length becomes much shorter than the synchrotron length.

Center frequency shift and reduction of feedback in directly modulated external cavity lasers

It is shown experimentally and theoretically that a center frequency shift occurs when an external cavity laser is directly modulated. The shift can be observed even when the frequency deviation is small compared to the roundtrip frequency of the external cavity and can qualitatively be explained by a reduction in the effective feedback level due to modulation. The frequency shift was measured as a function of modulation frequency and current, and frequency shifts up to 350 MHz were observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Frequency locking and quasiperiodicity in a modulated external cavity injection laser

We report the observation of frequency locking and quasiperiodicity in a modulated external cavity injection laser. The order of appearance of the frequency-locked regions follows a Farey sequence as the modulation frequency is scanned at a fixed value of the modulation depth. Quasiperiodic behavior is observed when the modulation frequency is incommensurate with the external cavity round-trip frequency. The observed behavior is found to be in good agreement with a rate equation model.

Hybrid mode-locking of an angled-stripe C 3 laser

An antireflection-coated angled-stripe two-section C3laser has been mode-locked in an external cavity by application of sinusoidal modulation at the cavity round-trip frequency to the section providing gain while the other section provides saturable loss. Pulses less than 20 ps in duration with low noise and background intensity are obtained subject to the relaxed operational tolerances characteristic of hybrid mode-locking.

A model of a diode laser actively mode-locked by gain modulation

A rate-equation analysis of a diode laser in an external resonator which is actively mode-locked by periodic modulation of the injection current is presented. A computer simulation of the transient process of pulse generation is developed. The coupled rate equations for electron and photon densities as functions of time and position within the diode are solved numerically. The effects of sinusoidal and pulsed injection current, spatial and temporal gain saturation, various resonator configurations, mismatch between modulation frequency and round-trip frequency, and varying laser parameter values are considered. A comparison of results with those of previous theories is presented. Optimum conditions for mode-locking are stated.

Bandwidth-limited picosecond pulse generation by hybrid mode-locking in a ring cavity GaAlAs laser

Bandwidth-limited picosecond pulses have been obtained using hybrid active/passive mode-locking in an external cavity semiconductor diode ring laser. Active mode-locking by application of sinusoidal RF modulation at the cavity round-trip frequency to a diode laser providing gain is supplemented by passive mode-locking in a second laser which is biased to provide saturable loss. Simultaneous streak camera and scanning Fabry-Perot interferometer measurements show that bandwidth-limited pulses of 25 Ps duration are obtained. Operation is not limited to near laser threshold, and frequency detuning sensitivity is improved when compared to the case of purely active mode-locking.

Interaction of a bistable injection laser with an external optical cavity

Experimental results on interactions of a bistable laser with an external optical cavity are presented. Switching of a bistable injection laser can be done by varying the amount of optical feedback. The optical switching is accompanied by a switching of the voltage across the absorber section. This can be utilized in digital optical disk readout. A bistable laser with an antireflection coating on one facet is more suitable for this task. No pulsations can be observed in a bistable laser with optical feedback if the absorber section is biased with a constant current source; but when it is biased with a voltage source, pulsation occurs at the external cavity round trip frequency. This indicates that even though the intrinsic absorption of the semiconductor material does not saturate easier than the gain, the presence of such absorptions in GaAs lasers can still produce pulsations when the electrical aspect is taken into account.

Active mode locking of double heterostructure lasers in an external cavity

Proton bombarded stripe geometry lasers have been actively mode locked in an external cavity consisting of a lens, a Fabry-Perot etalon, and a mirror. Under optimized conditions, pulses as short as 5.3 psec have been observed from measurement of the second harmonic autocorrelation function. The dependence of the pulse width on external parameters was studied in detail. The pulse width t1/2 decreases with increasing direct current. The functional dependence of the pulse width on current indicates that a saturable absorption mechanism is present in the laser which causes significant pulse shortening. The pulse width decreases with increasing rf current. The pulse width is a sensitive function of the frequency detuning of the applied rf current. The shortest pulses are observed when the applied frequency is less than the round trip frequency of the pulse in the cavity. The frequency dependence of the pulse width was obtained using a long cavity with multiple pulses in the cavity. The shortest pulses occur between 0.9 and 1.4 GHz. The Fabry-Perot etalon limits the emission to a 1.9-Å spectral band. At optimum mode locking the band is ∼30 Å below the peak of the multimode emission obtained without the etalon. The shift of the gain curve to lower enery indicates that additional loss is introduced by the feedback from the external cavity. The experimental results are shown to be in good agreement with a theoretical description of mode locking.